When God miraculously destroyed all the first-born of the
   Egyptians, (Ex 12:29,) he spared those of the Israelites;
   and, in commemoration of that event, he was pleased to
   appoint that all the first-born males "should be set apart
   unto himself."  (Ex 13:12-16.)  God is here pleased to
   relinquish this claim, and to appoint the whole tribe of Levi
   to attend his immediate service in their stead.

  41,45  8:16,18  18:6